This invention relates to improvements in a sliding member which is lubricated with fuel, for an automotive vehicle, and more particularly to the improvements in a fuel injection valve for an automotive vehicle, including a needle valve whose sliding section (in slidable contact with an opposite member) is coated with a particular hard carbon thin film so as to be high in durability reliability and realize a low friction coefficient.
Recently, requirements for improving fuel economy and exhaust gas emission control to automotive vehicles have become further stringent, and therefore sliding conditions at sliding sections which are lubricated with fuels become further severe in order to suppress friction at such sliding sections. It has been proposed as a measure to suppress the friction at the sliding sections, that a hard thin film of chromium nitride, titanium nitride or the like is formed at the sliding section of the fuel injection valve as disclosed in Japanese Patent Provisional Publication No. 7-63135, the entire disclosure of which is hereby incorporated by reference.
The largest merits of forming such a hard thin film resides in a point where a remarkably high surface hardness is obtained as compared with a surface treatment such as plating and a surface-hardening treatment such as a heat treatment. By applying such a hard thin film onto the sliding section, it is expected that a wear resistance can be greatly improved. Additionally, under lubrication, such a hard thin film can suppress the degradation of the surface roughness due to wear, and therefore it prevents an opposite member from wearing due to the degraded surface roughness and prevents a frictional force from increasing due to an increase in direct contact (metal contact) with the opposite member, thereby making it possible to maintain a lubricating condition at an initial state for a long time. Furthermore, since the hard thin film itself is hard, it can be possible to make the opposite member adaptable to the hard thin film, and accordingly it can be expected to provide a function to obtain a smoothened surface roughness. As a result, it can be expected that the surface roughness of the both the hard thin film and the opposite member are improved in the lubricating condition.
Now, it has been known that an amorphous carbon film such as a diamond-like carbon (DLC) film which is a kind of hard thin films is high in hardness itself and has a characteristic serving as a solid lubricant itself, so that it exhibits a remarkably low friction coefficient under no lubrication.
As microscopically viewed in lubricating oil, the sliding section is divided into a section where the hard thin film slidably contacts with the opposite member through an oil film, and another section where projections due to the surface roughness (shape) of both the hard thin film and the opposite member directly contact with the facing member making a metal contact. At the latter section where the metal contact is made, an effect of lowering the frictional force generated there can be expected similarly in case of no lubrication, by applying a DLC film at the section. In this regard, it has been investigated to apply the DLC film as a technique for lowering friction in an internal combustion engine.
However, a hard thin film formed by a PVD process or a CVD process is high in internal stress as compared with a surface treatment such as plating and remarkably high in hardness. Accordingly, if the hard thin film is applied to the sliding section of machine parts, the hard thin film tends to peel off from a base material or to form its crack. Concerning such peeling-off of the hard thin film, it has been proposed to soften the internal stress so as to make an improvement by providing a suitable intermediate layer taking account of adhesiveness between the hard thin film and the base material or by applying a multiple layer structure of the hard thin film.
In connection with formation of cracks in the hard thin film itself and peeling-off of the hard thin film due to the cracks, there have hardly been conventional techniques which improve the hard thin film to prevent them by regulating the surface roughness and shape of the hard thin film (particularly, a hard carbon thin film) and them of the opposite member. Only measures which have been hitherto proposed are to form a hard carbon thin film consisting of C, H, Si and inevitable impurities is formed at the surface of the sliding section, regulating the thickness and hardness of the hard carbon thin film as disclosed in Japanese Patent Provisional Publication No. 2002-332571.